Solder paste is typically maintained in a refrigerated state to preserve the integrity of its composition and to thereby prolong its useful shelf life. Refrigeration is needed because when solder paste is subjected to prolonged heat exposure, it typically suffers from chemical deterioration and physical suspension problems. Moreover, when solder paste deteriorates due to heat exposure, it often fails to meet expected performance standards as a bonding agent.
Consequently, the storage and handling of solder paste is an area which has come under increasing scrutiny in the drive to reduce variation in surface mount technology (SMT) manufacturing processes. Typically, a container of solder paste is stamped with a six-month date signaling the expiration of the solder paste's useful shelf life (i.e., the duration of time during which the freshness or integrity of the paste can be ensured, given proper handling of the paste). The duration of six months is based on a preestablished thermal forecast for the solder paste over the course of its life with a margin of safety provided.
The solder paste's actual shelf life, however, is dependent on the nature of temperature excursions that the paste actually experiences from the time it is manufactured to the time it is reflowed on a board. To preserve the quality of the paste, these temperature variations need to be monitored and controlled. Temperature variations are typically monitored by logging, over time, the temperature of the refrigerated environment where the solder paste is maintained.